Enhanced gas separations and zeolite compositions therefor

ABSTRACT

Pressure swing adsorption air and other gas separation operations are carried out using specific preferred zeolite adsorbent compositions based on the symmetry of the framework atoms and cations therein. Specific SiO 2  /Al 2  O 3  and cation/cation ratios for enhanced performance are identified. LiX adsorbent having a silica to alumina ratio on the order of 2.364 provides enhanced PSA performance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to gas separation operations. More particularly, it relates to enhanced air and other gas separation operations using preferred zeolitic adsorbents.

2. Description of the Prior Art

For a wide variety of commercial applications in which cryogenic air separation plants may not be economically feasible, pressure swing adsorption (PSA) systems are particularly suitable. For example, PSA systems have been used to supply high purity oxygen for various applications, such as chemical processing, steel mills, paper mills, and lead and gas production operations.

In PSA processing, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g., the nitrogen and oxygen components of air, is passed to the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The less readily adsorbable component, e.g., oxygen, passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized to a lower desorption pressure for desorption of the more readily adsorbable component, and its removal from the feed end of the bed prior to the introduction of additional quantities of the feed gas mixture for repressurization of the bed and adsorption of the more readily adsorbable component as cyclic adsorption-desorption-repressurization operations are continued in the bed.

Such PSA processing is commonly carried out in multi-bed adsorption systems, with each bed employing the PSA processing sequence on a cyclic basis interrelated to the carrying out of the processing sequence in the other beds of the adsorption system. In PSA systems for the recovery of high purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed and recovered from the feed end of the bed upon reduction of the pressure of the bed from the upper adsorption pressure to a lower desorption pressure level. PSA systems for the recovery of nitrogen product have likewise been based on the use of adsorbents that selectively adsorb nitrogen from air as the more readily adsorbable component thereof, although other PSA-nitrogen processes are based on the use of oxygen-selective adsorbents, such as various carbon adsorbent materials.

Early PSA air separation systems utilized two or three beds, with well known molecular sieves, e.g., 13X zeolite molecular sieve material, being used as the adsorbent therein. Such zeolitic molecular sieve material, and other such materials, e.g., 5A zeolite molecular sieve material, capable of selectively adsorbing nitrogen from air are equilibrium type adsorbents. In the use of such adsorbents, an adsorption front of the selectively adsorbed nitrogen is formed at the feed end of the bed, and advances toward the discharge, or oxygen product, end of the bed as a result of equilibrium conditions established in the bed of zeolite molecular sieve material between the more readily adsorbable nitrogen and the less readily adsorbable oxygen component of feed air.

While conventional zeolite molecular sieves can be used in PSA operations, specially modified materials can also be employed for improved performance, such as for the improved adsorption of nitrogen from feed air, and the recovery of oxygen or nitrogen as the desired product gas. Thus, the lithium cation forms of conventional zeolite X have been developed for use in PSA processing. Such lithium, i.e., LiX, adsorbent is found to exhibit a highly desirable capacity and selectivity for the adsorption of nitrogen from feed air or other streams containing less polar or less polarizable molecular species, such as oxygen.

LiX adsorbent materials proposed for PSA processing operations are the lithium cation forms of zeolite in which the framework SiO₂ /Al₂ O₃ molar ratio is from about 2.0 to about 3.0, preferably from 2.0 to 2.5, and in which at least about 88%, preferably at least 90%, more preferably at least 95%, of the AlO₂ -tetrahedral units are associated with lithium cations. The nitrogen adsorption properties of such highly exchanged forms of LiX were not predictable from the results obtainable using LiX materials in which 86 equivalent percent or less of the cations are lithium and the remainder are principally sodium ions. Such highly exchanged LiX materials are further described in the Chao patent, U.S. Pat. No. 4,859,217, which recognized that high lithium exchange was required for high nitrogen selectivity and that a 99% LiX (2.0) material had a higher nitrogen capacity than a 99% LiX (2.5) material, although no explanation was provided for this circumstance.

In the Coe patent, U.S. Pat. No. 4,481,018, it is disclosed that mixed cation-exchanged X zeolites and faujasites having a Si/Al ratio of about 1.0 to 1.2 (corresponding to a SiO₂ /Al₂ O₃ ratio of about 2.0 to 2.5) can be used for the separation of nitrogen from gas mixtures. The patent teaches a range of SiO₂ /Al_(O) ₃ ratios and cation compositions for improved gas separations, but does not specify exact SiO₂ Al/₂ O₃ ratios or cation compositions that will result in superior selectivities for the more readily adsorbable component of the feed mixture. Likewise, the patent does not recognize or teach which structural or compositional features will control selectivity in these adsorbent materials.

Sircar et al., U.S. Pat. No. 4,557,736, have described the use of calcium/strontium-exchanged X zeolites as improved adsorbents. The SiO₂ /Al₂ O₃ ratios for enhanced performance are not specified, but ranges are given for calcium, strontium and sodium cation levels. The resulting materials were reported to have higher nitrogen adsorption capacities, lower heats of nitrogen adsorption and improved selectivities relative to non-exchanged precursors.

Lithium exchange was also disclosed in the Coe patent, U.S. Pat. No. 4,925,460, which relates to lithium-exchanged chabazites for air separation. The patent specifies a Si/Al ratio of 2.1-2.8 (corresponding to a SiO₂ /Al₂ O₃ ratio of 4.2-5.6), and a range of lithium exchange levels equal to, or greater than, 65%. Calcium-exchanged chabazites for gas separation are described in the Coe et al. patent, U.S. Pat. No. 4,943,304, which relates to the separation of minor components from bulk gases, and not to air separation or air purification applications. A Si/Al ratio of 1.9-2.3 is disclosed, as well as a special composition of Si/Al ratio =2, cation siting=1, and a cation distribution=1. Both the framework Si/Al ratio and the cations' position and distribution were said to affect the nitrogen adsorption properties of the adsorbent, but the relationship between the Si/Al (SiO₂ /Al₂ O₃) ratio and cation composition to adsorbent sample selectivity, i.e., the composition and/or structure of preferred adsorbent compositions, was not recognized in said patent.

The Coe patent, U.S. Pat. No. 4,544,378, teaches that mixed cation forms of X-type faujasites are advantageous for air separation purposes. Separation factors, determined by a gas chromatography method, are shown to be related to levels of cation exchange and adsorbent sample activation conditions. While higher selectivities are attributed to higher levels of cation exchange in an X (2.5) zeolite, no connection is made to specific compositions or framework structures for enhancing the selective adsorption characteristics of the mixed cation forms of X-type faujasites.

The advantages of mixed cation zeolites for air separation applications have also been recognized in two recently issued patents. Chao, U.S. Pat. No. 5,174,979, teaches the use of lithium/alkaline with metal zeolites of the X and A framework structures. SiO₂ /Al₂ O₃ ratios of about 1.85-3.0 were disclosed for X structures, and ratios of about 1.85-4.0 were disclosed for A structures. For lithium/alkaline earth metal X zeolites, cation ratios of about 95:5-50:50 are disclosed, while cation ratios of about 10:90-70:30 are disclosed for lithium/alkaline earth A zeolites. The Coe patent, U.S. Pat. No. 5,152,813, discloses the use of exchanged X zeolites with a Si/Al ratio of equal or less than 1.5 (SiO₂ /Al₂ O₃ ratio of equal or less than 3.0), having at least binary exchange of lithium and calcium and/or strontium, with preferable ratios of 5-50% calcium and/or strontium ions and 50-90% lithium ions. As with previous disclosures referred to above, these two patents claim ranges of Si/Al (SiO₂ /Al₂ O₃) ratios and cation concentrations, but do not teach specific combinations of framework and cation compositions for the achieving of enhanced performance of zeolites in PSA gas separation operations.

While the art has thus made significant progress in the development of special adsorbents to improve air separation and other PSA gas separation operations, there is a need for further improvement in the adsorbent field. In particular, there is a need to develop PSA air and other gas separation operations utilizing specific preferred zeolite compositions to better satisfy the ever-increasing requirements of a variety of industrial applications for the desirable pressure swing adsorption technology. Such specific preferred zeolite compositions employed in such enhanced PSA gas separation operations will enable enhanced selectivities for the more readily adsorbable component to be achieved, and lower cost zeolite adsorbent compositions to be considered, so as to achieve substantial savings in the operation of practical commercial PSA systems.

It is an object of the invention, therefore, to provide enhanced PSA processing operations and special adsorbents for use therein.

It is another object of the invention to provide enhanced performance in PSA air and other gas separation operations using preferred zeolite adsorbents.

It is a further object of the invention to provide specific combinations of framework and cation compositions capable of superior zeolite performance in PSA gas separation operations.

With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

Enhanced PSA air and other gas separation operations are carried out using specific zeolite adsorbent compositions determined based on the symmetry of the framework atoms and cations included in the adsorbent structure. Adsorbents determined by such symmetry are found to have specific SiO₂ /Al₂ O₃ and cation/cation ratios. Such compositions have superior equilibrium selectivities over those of neighboring compositions in the desired PSA and other gas separation operations.

BRIEF DESCRIPTION OF THE DRAWING

The invention is hereinafter described in detail with reference to the accompanying drawings in which:

FIG. 1 is a schematic drawing illustrating a symmetric zeolite composition;

FIG. 2 is a schematic drawing illustrating an asymmetric zeolite composition;

FIG. 3 is a schematic representation of symmetric, semisymmetric and nonsymmetric β-cage Al distributions in a unit cell of a zeolite adsorbent structure;

FIG. 4 is a graphic representation of the relationship between the framework charge in unit cells and various symmetric and semisymmetric framework compositions;

FIG. 5a is a chart illustrating the nitrogen selectivity of three lithium-exchanged framework compositions at recited operating conditions;

FIG. 5b is a chart illustrating the nitrogen selectivity of said three framework compositions and different recited operating conditions;

FIG. 5c is a chart illustrating the nitrogen selectivity of said three framework compositions at different recited operating conditions;

FIG. 5d is a chart illustrating the nitrogen selectivity of said three framework compositions at different recited operating conditions;

FIG. 5e is a chart illustrating the nitrogen selectivity of said three framework compositions at different recited operating conditions;

FIG. 5f is a chart illustrating the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 6a is a chart showing the nitrogen selectivity of four calcium/lithium exchanged framework compositions at recited operating conditions;

FIG. 6b is a chart showing the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 6c is a chart showing the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 6d is a chart showing the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 6e is a chart showing the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 6f is a chart showing the nitrogen selectivity of said framework compositions at different recited operating conditions;

FIG. 7a is a chart illustrating the nitrogen selectivity of three CaNaX adsorbents having different framework compositions at recited operating conditions;

FIG. 7b is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 7c is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 7d is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 7e is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 7f is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 8a is a chart illustrating the nitrogen selectivity of MgLiX (2.0) adsorbents at a preferred cation composition and a non-preferred cation composition at recited operating conditions;

FIG. 8b is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 9a is a chart illustrating the nitrogen selectivity of CaLiX (2.0) adsorbent powders at two preferred and one non-preferred cation compositions at recited operating conditions;

FIG. 9b is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 10a is a chart illustrating the nitrogen selectivity of CaLiX (2.0) and CaX (2.0) adsorbent beads at different recited operating conditions;

FIG. 10b is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 10c is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 10d is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 10e is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 10f is a chart illustrating the nitrogen selectivity of said adsorbents at different recited operating conditions;

FIG. 11 is a chart illustrating the nitrogen selectivity of three different CaLiX (2.0) adsorbents and CaX (2.0) adsorbent at different recited operating conditions.

DETAILED DESCRIPTION OF THE INVENTION

The objects of the invention are accomplished by the discovery that PSA air and other gas separation operations can be enhanced using a pattern of preferred compositions for zeolitic adsorbents. This pattern is manifest in symmetrical considerations for framework structures and cation compositions of zeolites that can be used to identify preferred zeolite compositions for applications in gas separation, particularly air separation and air purification applications. As a result, specific SiO₂ /Al₂ O₃ and cation/cation ratios can readily be identified within the broad ranges disclosed in the prior art efforts to develop special adsorbents for PSA operations.

It has been known that lower SiO₂ /Al₂ O₃ ratios lead to more cations in an adsorbent structure, and, hence, higher nitrogen capacities. However, the relationship between framework symmetry and selectivity has not heretofore been understood in the art. While it has been shown in the art that cation exchange, particularly at high exchange levels, can lead to higher nitrogen capacities, and in some cases to higher selectivities for the more readily adsorbable component of a gas mixture, specific cation compositions of superior performance have not been discovered.

In the practice of the invention, the specific zeolite framework and cation compositions that will exhibit higher equilibrium selectivities than those of neighboring compositions can readily be determined. The use of such preferred zeolite compositions, providing technical and economic advantages in gas separation operations, is facilitated as the discovered pattern referred to above enables the specific preferred compositions to be determined without undue experimentation, and further enables lower cost zeolite compositions to be evaluated for any given gas separation application. Both of these features can result in substantial cost savings with respect to practical commercial PSA air or other gas separation operations.

The pattern of preferred compositions referred to above can be summarized as set forth below:

For zeolites of single or highly exchanged cation compositions:

1. Symmetric framework compositions will show higher equilibrium selectivities than neighboring non-symmetric framework structures.

2. Semisymmetric framework compositions will show higher equilibrium selectivities than neighboring non-symmetric framework structures.

3. Symmetric framework compositions will show higher equilibrium selectivities than neighboring semisymmetric framework structures.

For zeolites of mixed cation compositions:

1. Symmetric framework compositions with symmetric cation compositions will show higher equilibrium selectivities than neighboring symmetric framework compositions with non-symmetric cation compositions.

2. Semisymmetric framework compositions with semisymmetric cation compositions will show higher equilibrium selectivities than neighboring semisymmetric framework compositions with non-symmetric cation compositions.

3. For the same framework composition, symmetric cation compositions will show-higher equilibrium selectivities than neighboring semisymmetric cation compositions.

In order for the pattern of preferred compositions to be fully utilized in determining relative selectivities of various zeolite adsorbents for a particular application, comparisons should be limited to monovalent-monovalent vs. monovalent-monovalent, and divalent-divalent vs divalent-divalent, and monovalent-divalent vs monovalent-divalent comparisons.

The design principles for selecting and modifying zeolites to separate air are guided by certain general rules. In the case of equilibrium nitrogen-selective zeolites, structures having large pore volumes and large free pore diameters that are easy to produce are desired. Zeolite types A(LTA) and X(FAU) are desirable. To achieve higher nitrogen loadings, numerous exposed cations are also desired. In type X adsorbents, this implies low SiO₂ /A₂ O₃ ratios (limit =2.0). For single cation forms, cations with higher effective charge-to-radius ratios are preferred, so that Li⁺ is better than either Na⁺ or Ca⁺⁺. Mixed cation forms, e.g. CaLiX, can offer advantages to either of the end members. Such general rules are based on after-the-fact observations, and do not address questions about oxygen loadings, selectivity, and temperature effects. In addition, there is no basis in such general rules for selecting particular mixed cation compositions. Also, the rules are qualitative and need to be tested in the laboratory on a case-by-case basis. In order to advance the art, it is important to determine the relationships between structure and composition in zeolites, and their impact on adsorptive separations. The discovery of the invention addresses this need in the art.

β-Cages in Zeolites A, X and Y The structures of A, X and Y materials can be described in terms of β-cages, which are truncated octahedral arrays of tetrahedrally coordinated T-atoms, each bound to 4 oxygen atoms. Each β-Cage contains 24 T-atoms, either Al or Si for the zeolites of interest herein. The β-cages are attached to one another by groups of 4 O-atoms in A structures or of 6 in X or Y structures. The resulting β-cage networks define three-dimensional microporous systems of roughly spherical volumes called α-cages in A and "supercages" in X or Y structures. The latter are larger than the β-cages and are accessible through "windows" formed by rings of O-atoms. Adsorption of gases at pressures of interest for PSA operations occur in the α-cages or supercages.

The true unit cells of A(LTA) and of X and Y(FAU) each contain 8 β-cages, each of which is associated with an α-Cage or a supercage. In A type adsorbents, the β-cages are octahedrally coordinated, and the α-cages (free diameters=11.4 Å) have 6 windows with free diameters of 4.1 Å. In X and Y structures, the β-cages are tetrahedrally coordinated, and the supercages (free diameter=11.8 Å) have 4 windows with free diameters of 7.4 Å.

Distribution of Al atoms in unit cells--The zeolites of interest for adsorptive gas separations consist of interlinked chains of alternating T- and O-atoms, for example: --O--Si--O--Si--O--Al--O--Si--Al--O--Si--O--Al--O. The number of Al atoms per unit cell in A, X and Y is given by:

    N.sub.Al =192/(1+R.sub.Si/Al),

where "R_(Si/Al) " is the mole ratio of Si to Al atoms, as well as 1/2 the SiO₂ /Al₂ O₃ ratio. Note that "N_(Al) " decreases as "R_(Si/Al) " increases. The theoretical lower limit for the SiO₂ /Al₂ O)₃ ratio=2, which is a consequence of the empirical Lowenstein's Rule. This rule states that the distribution of Al in the chain --O--T_(i) --O--T_(j) --O--must be such that T_(i) and T_(j) cannot both be Al atoms.

Arrangement of Exchangeable Atoms--Each Al atom in the framework is associated with a net unit negative charge. To preserve electron neutrality in a unit cell with "N_(Al) " Al atoms, nonframework cations are present according to: N_(Al) =Σ[(n_(monovalent))+(2* n_(divalent))+(3*n_(trivalent))+. . . ], where n_(monovalent),n_(divalent), n_(trivalent), . . . are the number of cations with the indicated valence in the unit cell.

For each structure, the cations occupy crystallographically distinct sites on or within the β-cages or the interconnections. In X and Y, the number of cation sites ≧ number of cations. The various types of sites have different energies, which control the equilibrium distribution of cations among them. The problem of locating all of the cations in a singly exchanged X or Y has not been resolved, and even less is known about the locations of mixed cations. Some sites are within the structure so that cation size is important as well as the identity and charge.

In type A zeolites, the cations occupy sites in the center of the six-membered rings of the β-cages (64 per true unit cell), as well as sites in the windows of the α-cages. When there are more than 64 Al-atoms per unit cell (SiO₂ /Al₂ O₃ <4.00), there may be cations occupying sites in the windows. For example, monovalent ions such as Na⁺ and K⁺ (alone or in combination) in such sites may impede transport of N₂ relative to O₂ under some experimental conditions, leading to rate selectivity. The equilibrium selectivity of the invention is distinguished from such pore restriction effects.

Model or Adsorption in Zeolites

Basic Features--The zeolites of interest for equilibrium-selective adsorptive separations all contain microporous networks like those described above for the LTA and FAU topologies. In activated zeolites the gases are adsorbed in the α-cages or their analogs. The total pore volume is a rough measure of the adsorptive capacity so that large values are desirable. The effects of the shape of the α-cages or their analogs has not yet been determined. Those in A, X and Y are roughly spherical, while those in mordenite and chabazite are approximately cylindrical.

For air separation in N₂ -selective zeolites, the nitrogen capacity is related to the number of exposed cations, which, in turn, is controlled by the SiO₂ /Al₂ O₃ ratio. Cations for air separation have usually been selected from the alkali or alkaline earth elements, either singly or as mixed cations. Some of these cations have desirably high values of electrostatic field potential "z*e/r", the ratio of effective charge to radius. These cations also possess convenient chemical properties for synthesis and ion exchange. In X and Y, the smaller cations can fit in the shielded site I locations. Larger ones can occupy the shielded site I' positions. The relatively exposed positions are sites II, II' and III.

The adsorption of gas molecules is attributed to the interaction of the molecular properties of the gases with the fields produced by the cations. The framework atoms and their distributed negative charge also contribute to adsorption, but the separation is associated with differences in the molecule-cation interactions. In the case of air separation, the molecular properties of polarizability and electronic quadrupole moment are important. The polarizability of N₂ is about 10% larger than that of O₂, while the quadrupole moment of N₂ is 3.68 times that for O₂. The selectivity for N₂ is attributed to larger contributions to the adsorption energy for N₂ than for O₂ for the following electrostatic interactions: cation field with polarizability and cation field gradient with quadrupole moment. Until now, the focus of attention has been on interactions localized at one α-cage or supercage.

Implications for Separation--The equilibrium adsorption properties of types A and X in various cation forms for N₂, O₂ and their mixtures, can be expressed in different ways to reflect both the usefulness for separation and the adsorptive interactions from which this utility arises. One simple and convenient set of characteristic properties comprises ΔN₂ loadings, selectivities at feed and desorption pressures, and the enthalpies of adsorption. It will be appreciated that these can be readily calculated from pure gas isotherms for selected process conditions.

This set forms a convenient bridge between isotherm data and process parameters, such as bed size factor, power recovery and purity. For example, the bed size factor is related to the ΔN₂ loading, which is, in turn, controlled by the isotherm shape and the number and type of exposed cations. Power and recovery are related to the separation factors for the feed and desorption conditions, which are, in turn, controlled by the competitive adsorption of N₂ and O₂.

Competitive adsorption has been addressed in numerous theories, one of the earliest and most useful being the Loading Ratio Correlation. This formulation for mixed adsorption takes the view of the Langmuir isotherm. In it, N₂ and O₂ compete for a fixed number of adsorption sites, which have been identified in most theories as the exposed cations themselves. In the invention herein described and claimed, a much broader view, based on the unit cell, has been formed to identify symmetrical framework and cation compositions that possess higher selectivities than nonsymmetrical ones.

Discovery

Symmetrical Framework Compositions--Table 1 below recites a result of the study of the framework compositions for the unit cells of A, X and Y in terms of the number of Al atoms in the β-cages, in which the Al-atoms have been replaced by Si-atoms, one at a time, sequentially, in the 8 β-cages. The corresponding SiO₂ /Al₂ O₃ ratios are shown in the Table, starting from 2.000. Different structures span different ranges of =SiO₂ /Al₂ O₃ ratio: (1) for A, 2.0 ≧ ≧4.0; (2) for X, 2.0 ≧ ≧3.0; and (3) for Y, ˜3.0≧ ≧˜5.0, and for high silica FAU structures, ˜5.0≧ . Even though the space groups for A and X are different, they are both cubic and both have 8 β-cages in their respective unit cells.

                  TABLE 1                                                          ______________________________________                                         Distruibution of Al Atims in the Zeolite Framework                             Beta Cages in the Unit Cell                                                                      No. Al  Si/Al   SiO.sub.2 /Al.sub.2 O.sub.3                  #1  #2    #3    #4  #5  #6  #7  #8  Ions  Ratio Ratio                          ______________________________________                                         12  12    12    12  12  12  12  12  96    1.000 2.000                          11  12    12    12  12  12  12  12  95    1.021 2.042                          11  11    12    12  12  12  12  12  94    1.043 2.085                          11  11    11    12  12  12  12  12  93    1.065 2.129                          11  11    11    11  12  12  12  12  92    1.087 2.174                          11  11    11    11  11  12  12  12  91    1.110 2.220                          11  11    11    11  11  11  12  12  90    1.133 2.267                          11  11    11    11  11  11  11  12  89    1.157 2.315                          11  11    11    11  11  11  11  11  88    1.182 2.364                          10  11    11    11  11  11  11  11  87    1.207 2.414                          10  10    11    11  11  11  11  11  86    1.233 2.465                          10  10    10    11  11  11  11  11  85    1.259 2.518                          10  10    10    10  11  11  11  11  84    1.286 2.571                          10  10    10    10  10  11  11  11  83    1.313 2.627                          10  10    10    10  10  10  11  11  82    1.341 2.683                          10  10    10    10  10  10  10  11  81    1.370 2.741                          10  10    10    10  10  10  10  10  80    1.400 2.800                           9  10    10    10  10  10  10  10  79    1.430 2.861                           9   9    10    10  10  10  10  10  78    1.462 2.923                           9   9     9    10  10  10  10  10  77    1.494 2.987                           9   9     9     9  10  10  10  10  76    1.526 3.053                           9   9     9     9   9  10  10  10  75    1.560 3.120                           9   9     9     9   9   9  10  10  74    1.595 3.189                           9   9     9     9   9   9   9  10  73    1.630 3.260                           9   9      9    9   9   9   9   9  72    1.667 3.333                           8   9     9     9   9   9   9   9  71    1.704 3.408                           8   8     9     9   9   9   9   9  70    1.743 3.486                           8   8     8     9   9   9   9   9  69    1.783 3.565                           8   8     8     8   9   9   9   9  68    1.824 3.647                           8   8     8     8   8   9   9   9  67    1.866 3.731                           8   8     8     8   8   8   9   9  66    1.909 3.818                           8   8     8     8   8   8   8   9  65    1.954 3.908                           8   8     8     8   8   8   8   8  64    2.000 4.000                           7   7     7     7   7   7   7   7  56    2.429 4.857                           6   6      6    6   6   6   6   6  48    3.000 6.000                           5   5     5     5   5   5   5   5  40    3.800 7.600                           4   4     4     4   4   4   4   4  32    5.000 10.000                         ______________________________________                                    

For purposes of this invention, "symmetric framework compositions" are defined as those with an equal average number of the aluminum atoms per β-cage for all 8 in the true unit cell. These occur at intervals of 8 Al atoms replaced by Si-atoms, in the sequence of SiO₂ /Al₂ O₃ =2.000, 2.364, 2.800, 3.333, 4.000, 4.857, 6.000, . . . , as shown in said Table I. No attempt is made herein to account for the locations of the Al atoms in each β-cage, and order-disorder considerations arising from such siting are not addressed herein. "Semisymmetric framework compositions" are defined as those with an equal average number "m" of Al-atoms each in 4 of the β-cages of the true unit cell, and "m±1" Al atoms each in the other four βcages of each true unit cell, where 8m±4 is the total number of aluminum atoms in the true unit cell. These also occur at intervals of 8 Al-atoms replaced by Si-atoms, but in the sequence of SiO₂ /Al₂ O₃ =2,174, 2,571, 3.053, 3,647, 4.400, 5,385, 6,727, . . . , as shown in said Table 1.

It will be understood that the properties to be attributed to the special compositions of the invention will apply to a relatively narrow range of composition values on either side of those given above. Thus, a range corresponding to the replacement of ±1 Al-atom with a Si-atom pertains. In Table 2 below, some symmetric and semi-symmetric framework compositions of special interest, together with lower and upper limits thereof, are shown.

                  TABLE 2                                                          ______________________________________                                         PREFERRED                                                                      ZEOLITE FRAMEWORK SiO.sub.2 /Al.sub.2 O.sub.3 RATIOS                                    SYMMETRIC       SEMISYMMETRIC                                         ZEOLITE  COMPOSITIONS    COMPOSITIONS                                          TYPE     Low     Center  High  Low   Center                                                                               High                                ______________________________________                                         A        (1.96)  2.000   2.042                                                 (LTA)                          2.129 2.174 2.220                                        2.315   2.364   2.414                                                                                2.518 2.571 2.627                               X        (1.92)  2.000   2.042                                                 (FAU)                          2.129 2.174 2.220                                        2.315   2.364   2.414                                                                                2.987 3.053 3.120                               Y        3.260   3.333   3.408                                                 (FAU)                          3.565 3.647 3.731                                        3.908   4.000   4.095                                                                                4.295 4.400 4.508                                        4.737   4.857   4.982                                                                                5.245 5.385 5.529                               High     5.837   6.000   6.170                                                 Silica                         6.533 6.727 6.930                               X, Y     7.366   7.600   7.846                                                 (FAU)                          8.378 8.667 8.971                                        9.636   10.000  10.387                                                ______________________________________                                    

Symmetrical Compositions: Cations of a Single Type--"Symmetric" and "Semisymmetric" compositions for cations of a single type are defined, for purposes of this invention, in a manner analogous to symmetric framework compositions. For monovalent cations, the symmetric and semisymmetric compositions are those average values disclosed in Table 1, with upper and lower limits as shown in Table 2. As indicated above, limits have been defined by varying the cation composition ±1 cation from the ideal preferred compositions. Under these definitions, for divalent cations, there can be no symmetric values corresponding to odd number of Al-atoms per βcage, nor can there by any semisymmetric compositions. The symmetric divalent cation compositions occur at intervals of 16 Al-atoms replaced by Si-atoms, in the sequence of SiO₂ /Al₂ O₃ =2.000, 2.364, 2.800, 4,000, 6,000, . . . , as shown in Table 1 above.

Symmetrical Compositions: Mixed Cations--"Symmetric mixed cation compositions" are defined, for each symmetric framework composition, as those corresponding to the same average number of either monovalent cations, divalent cations, or mixtures thereof, of each type in the various combinations thereof being distributed in each β-cage of the true unit cell. The numbers of cations of different types associated with a particular βcage may be different one from another. As indicated above, no determination is made to account for the locations of the cations associated with each β-cage, and order-disorder considerations have not been made. "Semisymmetric binary cation compositions" are defined for each semisymmetric framework composition as those binary cation compositions for which: (1) all 8 β-cages have the same number of cations of one type, on the average, and (2) for the other cation type, "m" of monovalent cations, "m/2" of divalent cations, either separately or in combination of each type in the various combinations thereof being distributed in each of 4 of the 8 β-cages of the true unit cell, and "m±1" monovalent cations or "((m/2)+1)" divalent cations, either separately or in combination, with each type in the various combinations thereof being distributed in each of the other 4 β-cages, on the average, of the unit cell. Limits for both symmetric and semisymmetric mixed cation forms are set by varying the exchanged cation composition ±1 cation from the ideal preferred compositions.

There are several binary cation mixtures of special importance. Symmetric monovalent-monovalent compositions are presented in Table 3 below for SiO₂ /Al₂ O₃ =2.000 and 2,364.Semisymmetric monovalent-monovalent compositions are presented in Table 4 below for SiO₂ /Al₂ O₃ =2,571. The upper and lower limits for the values in Tables 3 and 4 are being presented in Table 5. Also shown in Table 5 are the semisymmetric monovalent-monovalent cation compositions with their respective upper and lower limits for the framework compositions SiO₂ /Al₂ O₃ =2,800, 3.053 and 3,333. Symmetric divalent-divalent compositions are shown in Table 6 for SiO₂ /Al₂ O₃ =2,000; with upper and lower limits being shown in Table 7. Symmetric monovalent-divalent compositions are set out in Table 8 for said ratio =2,000 and 2,364. Semisymmetric monovalent-divalent compositions are set out in Table 9 for SiO₂ /A₂ O₃ =2.571, with upper and lower limits being set out in Table 10.

                  TABLE 3                                                          ______________________________________                                         SYMMETRIC MIXED CATION COMPOSITIONS                                                                     No.    Equiv-                                                Beta Cages in the Unit Cell                                                                      Cat-   alent                                          Charges  #1    #2    #3  #4  #5  #6  #7  #8  tions                                                                               Fraction                     ______________________________________                                         For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000                            Monovalent                                                                              11    11    11  11  11  11  11  11  88   0.917                        Monovalent                                                                              1     1     1   1   1   1   1   1    8   0.083                        Monovalent                                                                              10    10    10  10  10  10  10  10  80   0.833                        Monovalent                                                                              2     2     2   2   2   2   2   2   16   0.167                        Monovalent                                                                              9     9     9   9   9   9   9   9   72   0.750                        Monovalent                                                                              3     3     3   3   3   3   3   3   24   0.250                        Monovalent                                                                              8     8     8   8   8   8   8   8   64   0.667                        Monovalent                                                                              4     4     4   4   4   4   4   4   32   0.333                        Monovalent                                                                              7     7     7   7   7   7   7   7   56   0.583                        Monovalent                                                                              5     5     5   5   5   5   5   5   40   0.417                        Monovalent                                                                              6     6     6   6   6   6   6   6   48   0.500                        Monovalent                                                                              6     6     6   6   6   6   6   6   48   0.500                        For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.364                            Monovalent                                                                              10    10    10  10  10  10  10  10  80   0.909                        Monovalent                                                                              1     1     1   1   1   1   1   1    8   0.091                        Monovalent                                                                              9     9     9   9   9   9   9   9   72   0.818                        Monovalent                                                                              2     2     2   2   2   2   2   2   16   0.182                        Monovalent                                                                              8     8     8   8   8   8   8   8   64   0.727                        Monovalent                                                                              3     3     3   3   3   3   3   3   24   0.273                        Monovalent                                                                              7     7     7   7   7   7   7   7   56   0.636                        Monovalent                                                                              4     4     4   4   4   4   4   4   32   0.364                        Monovalent                                                                              6     6     6   6   6   6   6   6   48   0.545                        Monovalent                                                                              5     5     5   5   5   5   5   5   40   0.455                        ______________________________________                                    

                                      TABLE 4                                      __________________________________________________________________________     SEMISYMMETRIC MIXED CATION COMPOSITIONS                                               Beta Cages in the Unit Cell                                                                            No. Equivalent                                  Charges                                                                               #1 #2 #3 #4 #5 #6 #7 #8 Cations                                                                            Fraction                                    For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.571                            __________________________________________________________________________     Monovalent                                                                            9  9  9  9  10 10 10 10 76  0.905                                       Monovalent                                                                            1  1  1  1  1  1  1  1   8  0.095                                       Monovalent                                                                            8  8  8  8  9  9  9  9  68  0.810                                       Monovalent                                                                            2  2  2  2  2  2  2  2  16  0.190                                       Monovalent                                                                            7  7  7  7  8  8  8  8  60  0.714                                       Monovalent                                                                            3  3  3  3  3  3  3  3  24  0.286                                       Monovalent                                                                            6  6  6  6  7  7  7  7  52  0.619                                       Monovalent                                                                            4  4  4  4  4  4  4  4  32  0.381                                       Monovalent                                                                            5  5  5  5  6  6  6  6  44  0.524                                       Monovalent                                                                            5  5  5  5  5  5  5  5  40  0.476                                       Monovalent                                                                            4  4  4  4  5  5  5  5  36  0.429                                       Monovalent                                                                            6  6  6  6  6  6  6  6  48  0.571                                       Monovalent                                                                            3  3  3  3  4  4  4  4  28  0.333                                       Monovalent                                                                            7  7  7  7  7  7  7  7  56  0.667                                       Monovalent                                                                            2  2  2  2  3  3  3  3  20  0.238                                       Monovalent                                                                            8  8  8  8  8  8  8  8  64  0.762                                       Monovalent                                                                            1  1  1  1  2  2  2  2  12  0.143                                       Monovalent                                                                            9  9  9  9  9  9  9  9  72  0.857                                       Monovalent                                                                            0  0  0  0  1  1  1  1   4  0.048                                       Monovalent                                                                            10 10 10 10 10 10 10 10 80  0.952                                       __________________________________________________________________________

                                      TABLE 5                                      __________________________________________________________________________     ZEOLITE MIXED CATION COMPOSITIONS                                              Monovalent-Monovalent Cation Combinations                                      (Equivalent Fraction of Second Cation)                                         LOW CENTER                                                                               HIGH                                                                               LOW CENTER                                                                               HIGH                                                                               LOW CENTER                                                                               HIGH                                     __________________________________________________________________________     (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.364)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.571)              0.073                                                                              0.083 0.094                                                                              0.080                                                                              0.091 0.102                                                                              0.083                                                                              0.095 0.107                                    0.156                                                                              0.167 0.177                                                                              0.170                                                                              0.182 0.193                                                                              0.179                                                                              0.190 0.202                                    0.240                                                                              0.250 0.260                                                                              0.261                                                                              0.273 0.284                                                                              0.274                                                                              0.286 0.298                                    0.323                                                                              0.333 0.344                                                                              0.352                                                                              0.364 0.375                                                                              0.369                                                                              0.381 0.393                                    0.406                                                                              0.417 0.427                                                                              0.443                                                                              0.455 0.456                                                                              0.464                                                                              0.476 0.488                                    0.490                                                                              0.500 0.510                                                                              0.534                                                                              0.545 0.557                                                                              0.560                                                                              0.571 0.583                                    0.573                                                                              0.583 0.594                                                                              0.625                                                                              0.636 0.648                                                  0.656                                                                              0.667 0.677             0.655                                                                              0.667 0.679                                                  0.716                                                                              0.727 0.739                                                  0.740                                                                              0.750 0.760                                                                                            0.750                                                                              0.762 0.774                                                  0.807                                                                              0.816 0.830                                                  0.823                                                                              0.833 0.844                                                                                            0.845                                                                              0.857 0.869                                                  0.898                                                                              0.909 0.920                                                  0.906                                                                              0.917 0.927                                                                                            0.940                                                                              0.952 0.964                                    (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.800)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 3.053)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 3.333)              0.088                                                                              0.100 0.113                                                                              0.092                                                                              0.105 0.118                                                  0.188                                                                              0.200 0.213             0.097                                                                              0.111 0.125                                                  0.197                                                                              0.211 0.224                                                  0.288                                                                              0.300 0.313             0.208                                                                              0.222 0.236                                                  0.303                                                                              0.316 0.329                                                  0.388                                                                              0.400 0.413             0.319                                                                              0.333 0.347                                                  0.408                                                                              0.421 0.434                                                  0.488                                                                              0.500 0.513             0.431                                                                              0.444 0.458                                                  0.513                                                                              0.526 0.539                                                  0.588                                                                              0.600 0.613             0.542                                                                              0.556 0.569                                                  0.618                                                                              0.632 0.645                                                  0.688                                                                              0.700 0.713             0.653                                                                              0.667 0.681                                                  0.724                                                                              0.737 0.750                                                  0.788                                                                              0.800 0.813             0.764                                                                              0.778 0.792                                                  0.829                                                                              0.842 0.855                                                  0.888                                                                              0.900 0.913             0.875                                                                              0.889 0.903                                                  0.934                                                                              0.947 0.961                                                  __________________________________________________________________________

                                      TABLE 6                                      __________________________________________________________________________     SYMMETRIC MIXED CATION COMPOSITIONS                                            Beta Cages in the Unit Cell  No. Equivalent                                    Charges                                                                             #1 #2 #3 #4 #5 #6 #7 #8 Cations                                                                            Fraction                                      For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000                            __________________________________________________________________________     Divalent                                                                            10 10 10 10 10 10 10 10 40  0.833                                         Divalent                                                                            2  2  2  2  2  2  2  2   8  0.167                                         Divalent                                                                            8  8  8  8  8  8  8  8  32  0.667                                         Divalent                                                                            4  4  4  4  4  4  4  4  16  0.333                                         Divalent                                                                            6  6  6  6  6  6  6  6  24  0.500                                         Divalent                                                                            6  6  6  6  6  6  6  6  24  0.500                                         Divalent                                                                            4  4  4  4  4  4  4  4  16  0.333                                         Divalent                                                                            8  8  8  8  8  8  8  8  32  0.667                                         Divalent                                                                            2  2  2  2  2  2  2  2   8  0.167                                         Divalent                                                                            10 10 10 10 10 10 10 10 40  0.833                                         __________________________________________________________________________

                  TABLE 7                                                          ______________________________________                                         ZEOLITE MIXED CATION COMPOSITIONS                                              DIVALENT-DIVALENT CATION COMPOSITION                                           (EQUIVALENT FRACTION OF SECOND CATION)                                         SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000                                            LOW           CENTER    HIGH                                                   ______________________________________                                         0.146         0.167     0.188                                                  0.313         0.333     0.354                                                  0.479         0.500     0.521                                                  0.646         0.667     0.688                                                  0.813         0.833     0.854                                                  ______________________________________                                    

                                      TABLE 8                                      __________________________________________________________________________     SYMMETRIC MIXED CATION COMPOSITIONS                                                   Beta Cages in the Unit Cell                                                                            No. Equivalent                                  Charges                                                                               #1 #2 #3 #4 #5 #6 #7 #8 Cations                                                                            Fraction                                    __________________________________________________________________________     For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000                            Monovalent                                                                            10 10 10 10 10 10 10 10 80  0.833                                       Divalent                                                                              2  2  2  2  2  2  2  2   8  0.167                                       Monovalent                                                                            8  8  8  8  8  8  8  8  64  0.667                                       Divalent                                                                              4  4  4  4  4  4  4  4  16  0.333                                       Monovalent                                                                            6  6  6  6  6  6  6  6  48  0.500                                       Divalent                                                                              6  6  6  6  6  6  6  6  24  0.500                                       Monovalent                                                                            4  4  4  4  4  4  4  4  32  0.333                                       Divalent                                                                              8  8  8  8  8  8  8  8  32  0.667                                       Monovalent                                                                            2  2  2  2  2  2  2  2  16  0.167                                       Divalent                                                                              10 10 10 10 10 10 10 10 40  0.833                                       For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.364                            Monovalent                                                                            9  9  9  9  9  9  9  9  72  0.818                                       Divalent                                                                              2  2  2  2  2  2  2  2   8  0.182                                       Monovalent                                                                            7  7  7  7  7  7  7  7  56  0.636                                       Divalent                                                                              4  4  4  4  4  4  4  4  16  0.364                                       Monovalent                                                                            5  5  5  5  5  5  5  5  40  0.455                                       Divalent                                                                              6  67 6                                                                              6  6  6  6  6  24 0.545                                           Monovalent                                                                            3  3  3  3  3  3  3  3  24  0.273                                       Divalent                                                                              8  8  8  8  8  8  8  8  32  0.727                                       Monovalent                                                                            1  1  1  1  1  1  1  1   8  0.091                                       Divalent                                                                              10 10 10 10 10 10 10 10 40  0.909                                       __________________________________________________________________________

                                      TABLE 9                                      __________________________________________________________________________     SEMISYMMETRIC MIXED CATION COMPOSITIONS                                               Beta Cages in the Unit Cell                                                                            No. Equivalent                                  Charges                                                                               #1 #2 #3 #4 #5 #6 #7 #8 Cations                                                                            Fraction                                    For Type X with SiO.sub.2 /Al.sub.2 O.sub.3 = 2.571                            __________________________________________________________________________     Monovalent                                                                            8  8  8  8  9  9  9  9  68  0.810                                       Divalent                                                                              2  2  2  2  2  2  2  2   8  0.190                                       Monovalent                                                                            6  6  6  6  7  7  7  7  52  0.619                                       Divalent                                                                              4  4  4  4  4  4  4  4  16  0.381                                       Monovalent                                                                            4  4  4  4  5  5  5  5  36  0.429                                       Divalent                                                                              6  6  6  6  6  6  6  6  24  0.571                                       Monovalent                                                                            2  2  2  2  3  3  3  3  20  0.238                                       Divalent                                                                              8  8  8  8  8  8  8  8  32  0.762                                       Monovalent                                                                            0  0  0  0  1  1  1  1   4  0.048                                       Divalent                                                                              10 10 10 10 10 10 10 10 40  0.952                                       __________________________________________________________________________

                                      TABLE 10                                     __________________________________________________________________________     ZEOLITE MIXED CATION COMPOSITIONS                                              Monovalent-Divalent Cation Combinations                                        (Equivalent Fraction of Second Cation)                                         LOW CENTER                                                                               HIGH                                                                               LOW CENTER                                                                               HIGH                                                                               LOW CENTER                                                                               HIGH                                     __________________________________________________________________________     (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.000)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.364)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 2.571)              0.146                                                                              0.167 0.188                                                                              0.159                                                                              0.182 0.205                                                                              0.167                                                                              0.190 0.214                                    0.313                                                                              0.333 0.354                                                                              0.341                                                                              0.364 0.386                                                                              0.357                                                                              0.381 0.405                                    0.479                                                                              0.500 0.521                                                                              0.523                                                                              0.545 0.568                                                                              0.548                                                                              0.571 0.595                                    0.646                                                                              0.667 0.688                                                                              0.705                                                                              0.727 0.750                                                                              0.738                                                                              0.762 0.786                                    0.813                                                                              0.833 0.854                                                                              0.856                                                                              0.909 no  0.929                                                                              0.952 no                                       (SiO.sub.2 /Al.sub.2 O.sub.3  = 2.800)                                                       (SiO.sub.2 /Al.sub.2 O.sub.3 = 3.053)                                                        (SiO.sub.2 /Al.sub.2 O.sub.3 = 3.333)              0.175                                                                              0.200 0.225                                                                              0.184                                                                              0.211 0.237                                                                              0.194                                                                              0.222 0.250                                    0.375                                                                              0.400 0.425                                                                              0.395                                                                              0.421 0.447                                                                              0.417                                                                              0.444 0.472                                    0.575                                                                              0.600 0.625                                                                              0.605                                                                              0.632 0.658                                                                              0.639                                                                              0.667 0.694                                    0.775                                                                              0.800 0.824                                                                              0.816                                                                              0.842 0.868                                                                              0.861                                                                              0.889 no                                       __________________________________________________________________________

Relation to Equilibrium Selectivity

Unit Cell Adsorption Values--Equilibrium selectivity in zeolites appears to be really based on competitive occupancy of relatively large adsorption volumes, not at individual cation sites. Prior investigations, which focused on the β-cages as the adsorption volume, suggest that the more strongly adsorbed component, e.g. N₂, will occupy regions close to the walls, and the cations, while the less strongly adsorbed component, e.g. O₂, will occupy regions near the center. This view of adsorption recognizes that adsorbed molecules do not "sit still" in cation sites, but are constantly in motion for the temperatures of interest in PSA separations. It is submitted that the β-cages or supercages are still too small to be considered as the "adsorption volumes". In Table 11, the number of molecules per supercage is given for 6 different zeolites at two temperatures for O₂ 26.7 kPa and for N₂ at 100 kPa. These data were calculated from pure gas isotherm data. The fractional values for oxygen, and the small number (≦4) for N₂, do not suggest competition at cation sites or in volumetric regions within the supercages.

                  TABLE 11                                                         ______________________________________                                         SUPERCAGE OCCUPANCIES (Molecules Per Supercage)                                          Oxygen          Nitrogen                                                       at 26.7 kPa     at 100 kPa                                           Zeolite     250K   300K       250K 300K                                        ______________________________________                                         NaX(2.3)    0.130  0.054      2.071                                                                               0.678                                       NaX(2.0)    0.110  0.045      1.962                                                                               0.604                                       LiX(2.5)    0.210  0.070      2.711                                                                               1.313                                       LiX(2.3)    0.197  0.072      2.949                                                                               1.382                                       LiX(2.0)    0.211  0.082      3.766                                                                               1.761                                       ______________________________________                                    

Rather than choose the adsorption volume intuitively or for convenience, it is desirable that the true unit cell be chosen as the adsorption volume, since it is the smallest volume that represents the zeolite crystal. In Table 12 below, the number of molecules per unit cell is given for the same zeolites and conditions as in Table 11. Since there are 8 α-cages in A, and 8 supercages in X or Y structures per unit cell, the occupancy numbers are 8 times higher. The adsorption volume is therefore properly viewed as distributed in space, so that the spatial probabilities for finding will extend over 8 α-cages in A or 8 supercages in X and Y. Thus for an N_(/2) O₂ mixture, an N₂ -rich adsorbate will be distributed in "pockets" in regions near the cations, and an O₂ -rich adsorbate will be distributed in regions near the centers of the α-cages or supercages.

                  TABLE 12                                                         ______________________________________                                         UNIT CELL OCCUPANCIES                                                          (MOLECULES PER UNIT CELL)                                                               Oxygen          Nitrogen                                                       at 26.7 kPa     at 100 kPa                                            Zeolite    250K   300K       250K  300K                                        ______________________________________                                         NaX(2.3)   1.042  0.429      16.571                                                                                5.427                                      NaX(2.0)   0.883  0.356      15.698                                                                                4.835                                      LiX(2.5)   1.682  0.561      21.688                                                                               10.502                                      LiX(2.3)   1.576  0.575      23.593                                                                               11.057                                      LiX(2.0)   1.691  0.660      30.130                                                                               14.089                                      ______________________________________                                    

Symmetry and Selectivity--It has been discovered that the selectivity for the more strongly adsorbable component, e.g. N₂, over the less strongly held component, e.g. O₂, in the gas mixture will be greater the higher the symmetry of the framework and cation compositions described earlier. If the number of charges associated with 1, 2 or 3 β-cages is different than the number in others in the unit cell, the field will then be distorted, and the less strongly held component (O₂) may approach closer to the cations, thereby competing more successfully with N₂. This effect is illustrated schematically in FIGS. 1 and 2 of the drawings. In FIG. 1, the symmetrical field keeps the O₂ -rich mixture in the "center" of the adsorption volume, while the asymmetrical field, as shown in FIG. 2, allows the O₂ -rich mixture of approach the cations. This symmetry effect is depicted schematically in another way in FIG. 3 of the drawings. In the illustrated embodiments, the cation charges associated with each β-cage are shown as integers at the apices of the cubes. It should be appreciated that this is not a representation of a crystal structure, but is just simply a way to show deviations from symmetry for a cubic array of 8 objects. The framework compositions corresponding to each array are also given in said FIG. 3.

Applications of the Discovery of the Invention

Usefulness of Symmetrical Compositions--It is known that both adsorption capacity and selectivity are required for practical PSA adsorbents. FIG. 4 of the drawings shows the relationship between the framework charge in the unit cell and several symmetric and semisymmetric framework compositions. In view of the strong dependence of N₂ sorption on exposed cations, it is reasonable to expect that .increment.N₂ loadings will show a similar dependence for a given cation composition. From FIG. 4 and Table 2, new N₂ -selective equilibrium adsorbents based on known zeolite forms can be determined. For air separation, preferred compositions of types X and A are particularly preferred. For air purification, preferred compositions of X and Y are preferred.

Illustrative Examples of Preferred Components of the Invention--The data herein have been obtained with a pressure microbalance under isothermal conditions for adsorbent samples activated under vacuum at 350° C. for about 16 hours. For a given sample, the adsorption isotherms for both N₂ and O₂ are mapped at 4 equilibrium points. The isotherm temperatures are 250, 273, 300 and 320K.

The data for each isotherm for each gas are fitted to a Loading Ratio Correlation (LRC) expression. LRC equations are used to calculate mixture adsorption data for competitive adsorption. Functions describing the separation are calculated at conditions chosen to sample some of those used in practical commercial PSA (O₂) cycles. The functions include measures of the N₂ sorbed and desorbed, and measures of selectivity at pressures and compositions corresponding to feed and desorption PSA processing steps. With respect to the illustrative examples below, these selectivity measures are employed, expressed as separation factors.

Evaluation of framework and cation compositions as preferred or non-preferred rely on symmetry, and narrow ranges, consistent with PSA performance, have been chosen to classify the "preferredness" or a particular composition. Limits for preferred framework compositions are, as indicated above, defined as the ranges corresponding to the replacement of ±1 Al-atom with an Si-atom for the ideal preferred compositions, which are, by definition, the centers of the ranges. These ranges have been outlined in Table 2 above for various framework types. Limits for both preferred and semi-symmetric mixed cation forms are likewise defined as the ranges corresponding to ±1 exchanged cation from the ideal preferred compositions, which are also defined as the centers of the ranges.

EXAMPLE 1

This example illustrates the concept of preferred compositions with regard to framework compositions. Three highly exchanged lithium X samples are compared at 24 conditions of temperature, pressure and N₂ /O₂ gas composition. The results are as shown in FIG. 5a, FIG. 5b, FIG. 5c, FIG. 5d, FIG. 5e and FIG. 5f of the drawings. The LiX (2.0) and LiX (2.3) structures have symmetric framework compositions, as noted above, while LiX (2.5) is a semisymmetric one. In all 24 cases, the symmetric framework compositions show higher separation factors than the semisymmetric ones, thus confirming the discovered pattern of preferred compositions.

It should be particularly noted that, under the particular conditions of FIG. 5a, FIG. 5b and FIG. 5c, at 300K operating temperature, the LiX (2.3) symmetrical material surprisingly enabled higher separation factor performance to be achieved than was obtained under like conditions using LiX (2.0) material.

EXAMPLE 2

The results obtained in this series of comparative operations is shown in FIG. 6a, FIG. 6b, FIG. 6c, FIG. 6d, FIG. 6e and FIG. 6f of the drawings, using CaLiX beads. The superior performance for symmetric framework compositions is shown in this illustrative example by comparison of four calcium-exchanged X zeolites at exchange levels of 10-25% calcium. At all 24 conditions tested, the symmetric framework examples, i.e. 15% CaLiX (2.0) and 17% CaLiX (2.3), exhibited higher separation factors than those of semisymmetric frameworks, i.e. 10% CaLiX (2.5) and 25% CaLiX (2.5). It is noted that, at the lower temperature, 250K, this effect is less pronounced.

This example also shows the effect of cation composition on performance. Table 10 above shows that the 17% CaLiX (2.3) and the 15% CaLiX (2.0) samples both fall in the ranges for preferred cation compositions, while the 10% and 25% CaLiX (2.5) samples fall outside these ranges. Thus, the superior performance of the 17% CaLiX (2.3) and the 15% CaLiX (2.0), as shown in FIGS. 6a-6f, can be attributed to both framework and cation effects.

It should also be noted that, in said examples summarized in FIGS. 6a-6f, the 2.3X material, i.e. 17% CaLiX (2.3), surprisingly exhibited higher separation factor performance at 273K than was achieved using the 2.0X material, i.e. 15% CaLiX, with the same PSA operating conditions.

EXAMPLE 3

In this example, the results of which are shown in FIGS. 7a-7f of the drawings, the interaction of preferred framework compositions and preferred cation compositions is shown. Three products based on a ˜75% CaNaX have been considered, namely CaNaX (2.0), CaNaX (2.3) and CaNaX (2.5). On the basis of preferred framework compositions, the separation factors obtained in PSA operations could be expected to decrease in the order of CaNaX (2.0) or CaNaX (2.3) >CaNaX (2.5). In fact, this progression is observed for the nine air feed cases at 250, 273 and 300K operating temperature and for the six N₂ -rich cases at 250 and 273K. From Table 10 above, it would appear that the selectivities obtained from the mixed cation compositions should decrease in the order CaNaX (2.5)>CaNaX (2.3)>CaNaX (2.0). This is observed for both gas compositions for the six cases at 320K. The relative selectivities of the three N₂ -rich gas cases at 300K appear to reflect the transition between the two regions. Thus, the relative importance of the two effects, framework and cation composition, depends on the operating temperature employed.

The results shown in FIG. 7b and in FIG. 7c indicate surprisingly superior performance for the 2.3 material at 300K for the indicated operating conditions.

EXAMPLE 4

This example illustrates the pattern of preferred compositions with regard to cation compositions. As shown in FIG. 8a and FIG. 8b of the drawings, the two illustrative compositions are both of the symmetric X (2.0) composition, but the 33.3% MgLiX (2.0) is a preferred cation composition as shown by Table 8 above, while the 26.5% MgLiX (2.0) is not. In all six cases tested, the preferred composition shows higher separation factors than the non-preferred one. It is clear that, for a given framework composition, the preferred cation composition can readily be identified by using the convenient methods described herein.

EXAMPLE 5

Preferred vs. non-preferred cation compositions are compared in this example, the results of which are shown in FIG. 9a and FIG. 9b. Two preferred (symmetric) cation compositions of the symmetric X (2.0) composition, i.e. 34.2% CaLiX (2.0) and 51.7% CaLiX (2.0), show higher separation factors than the highly exchanged CaX (2.0), a non-preferred (non-symmetric) cation composition (as shown in Table 10) in all six cases tested.

EXAMPLE 6

This example illustrates another case of preferred v. non-preferred cation compositions. The results are shown in FIGS. 10a-10f of the drawings, Both samples used in this example are of the symmetric X (2.0) composition, but the 15% CaLiX (2.0) sample is a preferred (symmetric) cation composition (Table 10), while the highly exchanged (non-symmetric) CaX (2.0) sample is not. In all 18 cases tested, the separation factors for the 15% CaLiX (2.0) beads exceed those of the CaX (2.0) beads, as expected. No data was available for CaX (2.0) at 250K.

EXAMPLE 7

This example, the results of which are shown in FIG. 11, was taken from data in the Chao patent, U.S. Pat. No. 5,174,976 and is used to compare preferred and non-preferred cation compositions. Selectivity is expressed as N₂ /O₂ loading ratios at 21% for equal N₂ and O₂ pressures. Four samples of the symmetric X(2.0) composition are compared: 15% CaLiX (2.0) (preferred; symmetric); 35% CaLiX (2.0) (preferred; symmetric); 54% CaLiX (2.0) (non-preferred; non-symmetric); and CaX (2.0) (non-preferred; non-symmetric). As expected, the N₂ /O₂ loading ratios for the two preferred cation compositions are higher than those of the non-preferred cation compositions. The 54% CaLiX (2.0) sample, which is a non-preferred structure, but close to the 47.9-52.1% preferred range (Table 10), shows loading ratios in all four pressure changes in between those of the two preferred compositions, 15% CaLiX (2.0) and 35% CaLiX (2.0) and the highly non-preferred (highly exchanged) CaX (2.0).

TYPE X FOR AIR SEPARATION

Preferred compositions, as will be appreciated from the above, are symmetric framework/symmetric cation compositions, symmetric framework/semisymmetric cation compositions, and semisymmetric framework/semisymmetric cation compositions. Most preferred compositions presently are LiX (2.000) and LiX (2.364) for air separation purposes, with LiX (2.364) being the best performing, most preferred of the two compositions. Other cation exchanged forms of symmetric X (2.364) and X (2.000) structures will also fall within the scope of the invention, with acceptable ranges for preferred framework and cation compositions of these and other materials being as disclosed herein. The symmetric composition X (2.800), may be of particular use at lower temperatures, e.g. in self-refrigerated cycles, especially where selectivity is more important than adsorptive capacity.

    ______________________________________                                         Type X Compositions for Air Separation at                                      Temperatures Near Ambient                                                      1. X (2.000)                                                                   a. Monovalent-Monovalent: Li.sup.+  and ≦2.0% Na.sup.+                  b. Monovalent-Divalent                                                         (1) Li.sup.+  and                                                              (2) Either Mg.sup.++ at compositions in Table 10                                 ≦68.8% Mg.sup.++                                                      (3) or Ca.sup.++  at compositions in Table 10                                    ≦52.1% Ca.sup.++                                                      2. X (2.364)                                                                   a. Monovalent-Monovalent: Li.sup.+  and ≦2.2% Na.sup.+                  b. Monovalent-Divalent                                                         (1) Li.sup.+ and                                                               (2) Either Mg.sup.++  at compositions in Table 10                                ≦75.0% Mg.sup.++                                                      (3) or Ca.sup.++ at compositions in Table 10                                     ≦56.8% Ca.sup.++                                                      COMPOSITIONS FOR TEMPERATURES                                                  BELOW AMBIENT                                                                  1. X (2.000)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦92.7% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦92.7% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦92.7% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦92.7% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦18.8% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦35.4% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦35.4% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦52.1% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++   at compositions in Table 10                         ≦52.1% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦68.8% Ca.sup.++                                                      2. X (2.364)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦92.0% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦92.0% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦92.0% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦92.0% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦20.5% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦38.6% Ca.sup. ++                                                     (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦38.6% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦56.8% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦56.8% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦75.0% Ca.sup.++                                                      3. X (2.800)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦91.3% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦91.3% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦91.3% K.sup.+                                                        (4) K.sup.+  and Na.sup.+   at compositions in Table 5                           ≦91.3% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦22.5% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦42.5% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦42.5% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦62.5% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦62.5% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦82.4% Ca.sup.++                                                      Type A Compositions for Air Separation at                                      Temperatures Near Ambient                                                      1. A (2.000)                                                                   a. Monovalent-Monovalent: Li.sup.+  and ≦2.0% Na.sup.+                  b. Monovalent-Divalent                                                         (1) Li.sup.+  and                                                              (2) Either Mg.sup.++  at compositions in Table 10                                ≦68.8% Mg.sup.++                                                      (3) Or Ca.sup.++  at compositions in Table 10                                    ≦52.1% Ca.sup.++                                                      2. A (2.364)                                                                   a. Monovalent-Monovalent: Li.sup.+  and ≦2.2% Na.sup.+                  b. Monovalent-Divalent                                                         (1) Li.sup.+  and                                                              (2) Either Mg.sup.++  at compositions in Table 10                                ≦75.0% Mg.sup.++                                                      (3) Or Ca.sup.++  at compositions in Table 10                                    ≦56.8% Ca.sup.++                                                      COMPOSITIONS FOR                                                               TEMPERATURES BELOW AMBIENT                                                     1. A (2.364)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦92.0% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦92.0% Li.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦20.5% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦38.6% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦38.6% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦56.8% Ca.sup.++                                                      Type X Compositions for Air Prepurification                                    1. X (2.800)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦91.3% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦91.3% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦91.3% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦91.3% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦22.5% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦42.5% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦42.5% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦62.5% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦62.5% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦82.5% Ca.sup.++                                                      2. X (2.571)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦96.4% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦96.4% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦96.4% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦96.4% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦21.4% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦40.5% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦40.5% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦59.5% Ca.sup. ++                                                     (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦59.5% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦78.6% Ca.sup.++                                                      3. X (3.053)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦96.1% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦96.1% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+  at compositions in Table 5                            ≦96.1% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦96.1% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦23.7% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup. ++  at compositions in Table 10                        ≦44.7% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦44.7% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦65.8% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦65.8% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦86.8% Ca.sup.++                                                      Type Y Compositions for Air Prepurification                                    1. Y (3.333)                                                                   a. Monovalent-Monovalent                                                       (1) Na.sup.+  and Li.sup.+  at compositions in Table 5                           ≦90.3% Li.sup.+                                                       (2) K.sup.+  and Li.sup.+  at compositions in Table 5                            ≦90.3% Li.sup.+                                                       (3) Na.sup.+  and K.sup.+   at compositions in Table 5                           ≦90.3% K.sup.+                                                        (4) K.sup.+  and Na.sup.+  at compositions in Table 5                            ≦90.3% Na.sup.+                                                       b. Monovalent-Divalent                                                         (1) Li.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦25.0% Mg.sup.++                                                      (2) Li.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦47.2% Ca.sup.++                                                      (3) Na.sup.+  and Mg.sup.++  at compositions in Table 10                         ≦47.2% Mg.sup.++                                                      (4) Na.sup.+  and Ca.sup.++  at compositions in Table 10                         ≦69.4% Ca.sup.++                                                      (5) K.sup.+  and Mg.sup.++  at compositions in Table 10                          ≦69.4% Mg.sup.++                                                      (6) K.sup.+  and Ca.sup.++  at compositions in Table 10                          ≦88.9% Ca.sup.++                                                      ______________________________________                                    

Various combinations of preferred framework and cation compositions of X, Y and A structures can be used in the practice of the invention. The combinations can be summarized as follows:

(1) Symmetric framework composition/symmetric cation compositions;

(2) Symmetric framework composition/semisymmetric cation compositions; and

(3) Semisymmetric framework composition/semisymmetric cation compositions.

Specific preferred framework compositions for X, Y and A structures are given in Table 2. Specific preferred cation compositions are given for monovalent-monovalent, divalent-divalent, and monovalent-divalent exchange in Tables 5, 7 and 10, respectively.

Cation combinations for monovalent-monovalent exchanges are desirably taken from the groups:

(a) H⁺, Li⁺, Na⁺, and K⁺, and

(b) H⁺, Li⁺, Na⁺, and K⁺,

For divalent-divalent exchange, combinations are desirably taken from the groups:

(a) Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, and Ba⁺⁺,and

(b) Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, and Ba⁺⁺.

For monovalent-divalent exchanges, combinations are desirably taken from the groups:

(a) H⁺, Li⁺, Na⁺, and K⁺, and

(b) Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, and Ba⁺⁺.

It will be understood that the selection of cations for the present invention should avoid certain combinations of larger ions such as K⁺, Sr⁺⁺, Ba⁺⁺, especially in the A structure, where rate selectivity occurs due to blockage of the α-cage windows.

It will be understood that various changes and modifications can be made in the details of the invention as described herein without departing from the scope of the invention as set forth in the appended claims. It will be appreciated that the zeolites of preferred compositions described herein can be prepared by known hydrothermal synthesis of the preferred framework structure, followed by ion exchange if needed. Typically the framework structure will be prepared in its alkali metal form, e.g. sodium X. In some cases, it may be necessary to convert one alkali metal form to another, for example, potassium X to sodium X, as taught by Chao in U.S. Pat. No. 4,859,217.

Typical synthetic routes for the framework structures of X, Y, A and high silica X and Y include an alumina source, a silica source, a hydroxide source and water. The stoichiometry of the final product is determined by the nature and properties of the reactants and the crystallization conditions employed in accordance with conventional methods well known in the art. The resulting zeolite powders can be ion exchanged directly, but for PSA applications, the powders are usually aggregated in beaded or extrudate form using commonly available binders.

The binders used to aggregate the zeolites may include clays, silica, alumina metal oxides and mixtures thereof. In addition, the zeolites may be formed with materials such as silica, alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-theria, silica-berylia and silica-titania, as well as ternary compositions, such as silica-alumina-theria, silica-alumina-zirconia and clays present as binders. The relative proportions of the above materials and the zeolites may vary widely with the zeolite content ranging from about 1 to about 99% by weight of the composite. When the zeolite is to be formed into aggregates prior to use, such aggregates are desirably about 1 to about 4 mm in diameter.

Ion exchange is performed on the framework zeolites by contacting the powders or aggregated forms with an aqueous solution of the metal salt. As is well known in the art, the ion concentration, reaction temperature and pH conditions employed will control the final composition of the product. In the absence of established ion exchange isotherms, empirical methods are used to obtain exact cation compositions. Such ion exchange techniques are well known and widely practiced in the art, and are disclosed, for example, in patents such as Chao et al., U.S. Pat. No. 5,174,979, Chao, U.S. 4,859,217, and Coe et al., U.S. Pat. No. 5,152,813.

It is also known in the art that proper thermal activation of molecular sieves is used to attain optimum performance. A thermal activation overnight at about 350° C. under vacuum conditions, with the temperature being raised slowly over several hours, commencing with ambient temperature, is generally sufficient. Examples of various activation conditions are also disclosed in the three patents referred to above.

In addition to air separation, for the production of oxygen or nitrogen, and air prepurification, for the removal of water, CO₂ or other impurities, the invention can also be used for various other gas separation processes in which the separation of major or minor constituents from bulk gas streams is desired. The major or minor constituents could be nitrogen, methane, carbon monoxide, carbon dioxide, or mixtures thereof, while the bulk gases could be argon, hydrogen, helium, krypton, neon, or mixtures thereof.

The practice of the invention enables air separation and other commercially significant PSA gas separation operations to be carried out advantageously using preferred zeolitic adsorbent compositions. The symmetrical features for framework compositions and cation compositions of preferred zeolite adsorbents enables preferred adsorbents to be selected for desirable PSA gas separation operations, limiting the number of experiments needed to optimize a particular gas separation operation and enabling lower cost compositions to be advantageously employed, resulting in substantial savings in any given air separation or other important PSA gas separation operation. 

We claim
 1. An enhanced process for the selective adsorption of a more readily adsorbable component of a gas mixture containing said component and a less readily adsorbable component in an adsorption system containing at least one bed of adsorbent material capable of selectively adsorbing the more readily adsorbable component from the gas mixture, each bed in said adsorption system undergoing, on a cyclic basis, a processing sequence comprising:(a) introducing the gas mixture at an upper adsorption pressure to said bed containing adsorbent material comprising an X or A type zeolitic adsorbent composition having true unit cells of 8 β-cages, each true unit cell having a symmetrical framework composition and a symmetrical cation composition, said symmetrical cation composition being monovalent, divalent, or mixtures thereof, said symmetrical framework composition having an equal average number of Al atoms per β-cage in all 8 β-cages of each true unit cell and said symmetrical cation composition having the same average number of either monovalent or divalent cations, or of mixtures of each type, distributed in each β-cage of the true unit cell, said symmetrical framework composition having a SiO₂ /Al₂ O₃ ratio of about 2.364; (b) depressurizing the bed by the release of gas therefrom so as to decrease the pressure in the bed from said upper adsorption pressure to a lower desorption pressure so as to desorb said more readily adsorbable component and pass it from the bed; (c) repressurizing the bed from the lower desorption pressure to said upper adsorption pressure; and (d) introducing additional quantities of the feed gas mixture to the bed as the cyclic processing sequence is carried out in each bed in the adsorption system, whereby enhanced equilibrium selectivity for the adsorption of the more readily adsorbable component of the gas mixture is achieved.
 2. The process of claim 1 in which said gas mixture comprise air.
 3. The process of claim 1 in which said zeolitic adsorbent composition comprises LiX.
 4. The process of claim 1 in which said zeolitic adsorbent composition comprises CaLiX.
 5. The process of claim 1 in which said zeolitic adsorbent composition comprises CaNaX.
 6. The process of claim 1 in which said zeolitic adsorbent composition comprises MgLiX.
 7. An X or A type zeolite adsorbent composition capable of enhanced selectivity for the adsorption of a more readily adsorbable component from a gas mixture and having true unit cells of 8β-cages, each true unit having a symmetrical framework composition and a symmetrical cation composition, said symmetrical cation composition being monovalent, divalent, or mixtures thereof, said symmetrical framework composition having the same average number of Al atoms per β-cage in all 8β-cages of each true cell unit and said symmetrical cation composition having the same average number of either monovalent or divalent cations, or of mixtures of each type, distributed in each β-cage of the true unit cell, said symmetrical framework composition having a SiO₂ /Al₂ O₃ ratio of about 2.364, whereby said zeolite adsorbent composition is capable of exhibiting enhanced selectivity for the more readily adsorbable component of a gas mixture.
 8. The zeolite adsorbent of claim 7 in which said zeolite adsorbent comprises LiX adsorbent.
 9. The zeolite adsorbent composition of claim 7 in which said zeolite adsorbent comprises CaLiX adsorbent.
 10. The zeolite adsorbent composition of claim 7 in which said zeolite adsorbent comprises CaNaX adsorbent.
 11. The zeolite adsorbent composition of claim 7 in which said zeolitic adsorbent comprises MgLiX adsorbent. 